Method for producing mixtures consisting of diphenylmethane diisocyanates and polyphenylene-polymethylene-polyisocyanates containing a reduced amount of chlorinated secondary products and with a reduced iodine color index

ABSTRACT

In a process for preparing mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates having a reduced content of chlorinated by-products and a reduced iodine color number by two-stage reaction of the corresponding mixtures of diphenylmethanediamines and polyphenylpolymethylenepolyamines with phosgene in the presence of at least one inert organic solvent at elevated temperature, separation of the excess phosgene and solvent after the phosgenation is complete and thermal treatment of the reaction product, the mass ratios of phosgene to hydrogen chloride in the residence time apparatus of the second stage of the phosgenation are at the same time 10-30:1 in the liquid phase and 1-10:1 in the gas phase.

The present invention relates to a process for preparing mixtures ofdiphenylmethane diisocyanates and polyphenylpolymethylenepolyisocyanates, known as PMDI, having a reduced content of chlorinatedby-products and a reduced iodine color number by two-stage reaction ofthe corresponding mixtures of diphenylmethanediamines andpolyphenylpolymethylenepolyamines, known as PMDA, with phosgene in thepresence of at least one inert organic solvent, where the correspondingcarbamoyl chlorides formed in the first stage of the phosgenation andthe amine hydrochlorides in the second stage of the phosgenation runthrough a residence time apparatus in which the amine hydrochlorides arephosgenated to the corresponding carbamoyl chlorides and the carbamoylchlorides are dissociated into the corresponding isocyanates andhydrogen chloride and the mass ratios of phosgene to hydrogen chlorideare at the same time 10-30:1 in the liquid phase and 1-10:1 in the gasphase.

PMDI is the industrially most important isocyanate for producing rigidpolyurethane foams which are preferably used as insulation material inthe building industry, as insulating foam in the refrigeration applianceindustry and as sandwich construction material. Usually, part of thediphenylmethane 4,4′-diisocyanate, known as MMDI, present in the PMDI isrecovered by means of a suitable technological operation such asdistillation or crystallization. MMDI is in turn an importantconstituent of polyurethane formulations for compact, microcellular andcellular polyurethanes such as adhesives, coatings, fibers, elastomersand integral foams. Accordingly, the term “PMDI” used in the presenttext also encompasses PMDI mixtures in which monomeric MDI, for example4,4′-, 2,2′- and/or 2,4′-MDI, is present.

PMDI is, as is known, prepared by phosgenation of the corresponding PMDAin the presence of an inert organic solvent. PMDA is in turn obtained bymeans of an acid aniline-formaldehyde condensation which can be carriedout industrially either continuously or batchwise. The proportions ofdiphenylmethanediamines and the homologouspolyphenylpolymethylenepolyamines and their positional isomerism in thePMDA are controlled by selection of the ratios of aniline, formaldehydeand acid catalyst and also by means of a suitable temperature andresidence time profile. High contents of 4,4′-diphenylmethanediaminetogether with a simultaneously low proportion of the 2,4′ isomer ofdiphenylmethanediamine are obtained on an industrial scale by the use ofstrong mineral acids such as hydrochloric acid as catalyst in theaniline-formaldehyde condensation.

All the acid aniline-formaldehyde condensation processes described inthe specialist and patent literature have in common the formation ofundesired by-products, for example the formation of N-methylated andN-formylated compounds and also the formation of dihydroquinazolines. Inaddition, industrial PMDAs can contain residual amounts of unrearrangedaminobenzylanilines which can in turn be a further starting point forfurther reactions. Another disadvantage is that the acidaniline-formaldehyde condensation forms chromophores which discolor thePMDA. These discolorations are reduced only insufficiently, if at all,in the subsequent neutralization of the acid condensation catalyst andthe removal of the aniline used in excess in the condensation; the sameapplies to the subsequent process steps of the PMDI preparation.

In the phosgenation step, the PMDA is reacted with phosgene in an inertorganic solvent to form PMDI. The undesired by-products and chromophoresin the PMDA can react with phosgene to form further compounds such assecondary carbamoyl chlorides and products of chlorination of thearomatic ring and/or at the methylene bridge. In addition, thephosgenation step forms further chlorine-containing by-products such asallophanoyl chlorides and isonitrile dichlorides. Thechlorine-containing compounds and chromophores are incorporated bothinto the low molecular weight fraction whose central constituent is thediphenylmethane diisocyanate and also into the oligomeric fractions ofpolyphenylpolymethylene polyisocyanate.

The technological operations which follow the phosgenation, namelyremoval of the phosgene used in excess, the removal of the inertsolvent, the thermal treatment, the so-called dechlorination and theremoval of part of the MMDI present in the crude PMDI by distillationand/or crystallization, do not lastingly reduce the content ofchlorine-containing compounds and the discoloration of the crude PMDIincreases with continuing, especially thermal, stressing of the product.

Chlorine-containing and/or discolored PMDI is undesirable in furtherprocessing to form polyisocyanate-polyalcohol polyaddition plastics. Inparticular, chlorine-containing compounds which can readily form ionicchloride, as determined by the ASTM D 1638-74 method, can causeconsiderable interference in the blowing reaction of foam production byforming salts with the blowing catalyst. Undesirable discolorations ofthe PMDI also show up in the plastics prepared therefrom. Although thecolor of the polyisocyanate-polyalcohol polyaddition plastics does nothave an adverse effect on their mechanical properties, light-coloredproducts are preferred because of their good versatility in theproduction process of the processor, e.g. the ability of light to passthrough thin covering layers and the ability to produce a variety ofcolors.

There have therefore been many attempts to reduce the content ofchlorinated by-products and the discoloration of PMDI in mixtures withMMDI.

According to GB 1 549 294, addition of isoureas in an amount of 25-250mol % can reduce the ASTM D 1638-74 acidity of the PMDI. A disadvantageof this method is that an additional agent has to be used and thelowering of the acidity is only partially successful.

DD 285 593 proposes treating PMDI with acid amides in an amount of0.01-0.2% at 100-140° C. for 0.2-6 hours. After the treatment, thehydrogen chloride formed is driven off by stripping with nitrogen orsolvent vapors. Disadvantages of this process are the insufficienteffect of the acid amides, the formation of additional constituents inthe PMDI as a result of the unavoidable secondary reaction of theisocyanates with the acid amides to form acylated ureas and the outlayin terms of apparatus for treating the PMDI with the acid amides and forstripping out the hydrogen chloride, both that added as catalyst andthat which is formed.

DE 2 847 243 proposes removal of phosgene by stripping with gaseoushydrogen chloride or nitrogen at 170° C. for 2 hours. A disadvantage isthe considerable amounts of gases laden with phosgene or withphosgene/hydrogen chloride which make an additional outlay for thesubsequent materials separation or an additional outlay for theneutralization of the acidic gas constituents absolutely necessary. Theadditional disadvantage of the process described in DE 2 847 243, namelythe long residence time for stripping, is partially alleviated in JP 07233 136 A by two-stage stripping with hydrogen chloride after phosgeneremoval at 115° C./30 minutes and 160° C./3 minutes. However, thisresults in the disadvantage of an additional technological operation andan again significant gas stream which requires treatment.

According to JP 07 082 230 A, organic phosphites are added to theaniline before the aniline-formaldehyde condensation.

To lower the iodine color number, the addition of numerous compoundsafter the phosgenation has been proposed: water (U.S. Pat. No.4,465,639), phenol derivatives (DE 4 300 774), amines and/or ureas (DE 4232 769), acid chlorides/chloroformates (DE 4 118 914), polyoxyalkylenepolyalcohols (DE 4 021 712), dialkyl or trialkyl phosphites (DE 4 006978), low molecular weight monohydric or polyhydric alcohols (EP 445602), acid chlorides/antioxidant (DE 4 318 018).

All processes which propose the addition of compounds to raw materialsor products of a preparation stage for PMDI have the disadvantage of theaddition of an additional agent with the inherent danger of itscorrosive action on the equipment components and the formation ofby-products from precisely these added agents, which by-products can inturn have an adverse effect on the product or the equipment.

U.S. Pat. No. 4,876,380 proposes lightening the color by extraction of achromophore-rich PMDI fraction from the PMDI by means of pentane/hexane.Disadvantages of this process are the carrying-out of a complicatedtechnological operation with additional steps for working up theextractant and the unavoidable formation of a reduced-quality PMDIfraction for which applications that use up equivalent amounts have tobe found.

It is an object of the present invention to reduce the content ofchlorinated by-products and the iodine color number of the PMDI inadmixture with MMDI while avoiding the abovementioned disadvantages. Inparticular, the addition of auxiliaries and/or the use of additionalapparatuses should not be necessary.

We have found that this object is achieved by two-stage reaction of thecorresponding mixtures comprising diphenylmethanediamines andpolyphenylpolymethylenepolyamines with phosgene in the presence of atleast one inert organic solvent, where the corresponding carbamoylchlorides formed in the first stage of the phosgenation and the aminehydrochlorides in the second stage of the phosgenation run through aresidence time apparatus in which the amine hydrochlorides arephosgenated to the corresponding carbamoyl chlorides and the carbamoylchlorides are dissociated into the corresponding isocyanates andhydrogen chloride and the mass ratios of phosgene to hydrogen chlorideare at the same time 10-30:1 in the liquid phase and 1-10:1 in the gasphase.

The present invention accordingly provides a process for preparingmixtures comprising diphenylmethane diisocyanates andpolyphenylpolymethylene polyisocyanates having a reduced content ofchlorinated by-products and a reduced iodine color number by two-stagereaction of the corresponding mixtures comprisingdiphenylmethanediamines and polyphenylpolymethylenepolyamines withphosgene in the presence of at least one inert organic solvent, whereinthe corresponding carbamoyl chlorides formed in the first stage of thephosgenation and the amine hydrochlorides in the second stage of thephosgenation run through a residence time apparatus in which the aminehydrochlorides are phosgenated to the corresponding carbamoyl chloridesand the carbamoyl chlorides are dissociated into the correspondingisocyanates and hydrogen chloride and the mass ratios of phosgene tohydrogen chloride are at the same time 10-30:1 in the liquid phase and1-10:1 in the gas phase.

The phosgenation of primary amines in a mixing reactor as first stage ofthe phosgenation has been described a number of times. Thus, forexample, U.S. Pat. No. 3,544,611 and EP A2-0150435 report thephosgenation in a pressure mixing circuit. Furthermore, EP A2-0291819discloses carrying out this reaction in a reaction pump. Many differentdesigns of static mixers have been described, for example: annular slotnozzle (FR 2 325 637, DE 1 792 660), ring-eye nozzle (DE 3 744 001),flat jet nozzle (EP A1-0 065 727), fan jet nozzle (DE 2 950 216),angle-jet chamber nozzle (DD 300 168), three-fluid nozzle (DD 132 340).

It is known per se that the corresponding carbamoyl chlorides and aminehydrochlorides formed in the first stage of the phosgenation can be runthrough a residence time apparatus in which the amine hydrochlorides arephosgenated to form the corresponding carbamoyl chlorides and thecarbamoyl chlorides are dissociated into the corresponding isocyanatesand hydrogen chloride. The isocyanate prepared according to WO 96/16 028in a tube reactor at 80-150° C. has a very unsatisfactory hydrolyzablechlorine content of max. 2% and makes PMDI prepared by this processunusable for most applications. In BE 790 461 and BE 855 235, stirredapparatuses are used as residence time reactors. U.S. Pat. No. 3,544,611describes a distillation residence time apparatus operating at 10-50 barand 120-150° C. and having an “elongated distillation zone” fordissociating the carbamoyl chlorides and removing the hydrogen chloride.DE 3 744 001 proposes a perforated plate column through which thereaction mixture flows from the bottom upward and which has more than 10perforated plates, a residence time of max. 120 minutes and liquidvelocities of 0.05-4 m/s and gas velocities of 2-20 m/s. Disadvantagesof the prior art are the drastic conditions in the residence timeapparatuses and the relatively long residence time of the crude PMDIformed. Experience indicates that the prior art allows only a veryunsatisfactory quality level in respect of the color and the chlorinecontent of the PMDI.

The combination of mixing and residence time apparatuses for preparingPMDI, in particular for the two-stage phosgenation, is also known. Thus,in DE 3 744 001, a ring-eye nozzle as reactor for reacting primaryamines with phosgene in an inert solvent to give the correspondingcarbamoyl chlorides and amine hydrochlorides is combined with one ormore perforated plate columns as apparatus for phosgenating the aminehydrochlorides and dissociating the carbamoyl chlorides. In U.S. Pat.No. 3,381,025, the first stage is carried out at <60° C. in an inertsolvent having a boiling point of 100-190° C. and the reaction productis transferred to a second stage in which the temperature is held atsuch a level above the boiling point of the inert solvent that the ratioof escaping phosgene to inert solvent is greater than two and, ifdesired, phosgene is additionally fed into the second reaction stage.Disadvantages are the high outlay in terms of apparatus and the highenergy consumption in the second stage of the phosgenation as residencetime apparatus or for condensing the gaseous mixture of phosgene/inertsolvent. Experience indicates that the prior art allows only a veryunsatisfactory quality level in respect of the chlorine content and thecolor of the PMDI.

It is therefore a further object of the present invention to reduce thecontent of chlorinated by-products and the iodine color number of PMDIusing technological equipment which is simpler in terms of safety andapparatus.

We have found that this object is achieved by two-stage reaction of PMDAwith phosgene in the presence of at least one inert organic solvent,where the first stage of the phosgenation is carried out in a staticmixer and the second stage of the phosgenation is carried out in aresidence time apparatus and the mass ratios of phosgene to hydrogenchloride in the residence time apparatus are at the same time 10-30:1 inthe liquid phase and 1-10:1 in the gas phase.

Static mixers employed for the first stage of the phosgenation are theknown and abovementioned pieces of equipment, in particular nozzles. Thetemperature in the first stage of the phosgenation is usually from 40 to150° C., preferably from 60 to 130° C., particularly preferably 90-120°C.

The mixture from the first stage of the phosgenation is fed to a columnin which, according to the present invention, the mass ratios ofphosgene to hydrogen chloride in the second stage of the phosgenationare at the same time 10-30:1 in the liquid phase and 1-10:1 in the gasphase.

It is particularly advantageous to operate the column in countercurrent.The product mixture from the first stage of the phosgenation ispreferably fed into the column such that the PMDI/solvent/phosgenemixture leaves the column at the bottom and a phosgene/hydrogen chloridemixture is taken off at the top of the column and is fed to the hydrogenchloride/phosgene separation. The temperature at which the mixture fromthe first stage of the phosgenation enters the column is preferably80-120° C., particularly preferably 82-117° C. The temperature at thebottom of the column is preferably 80-120° C., particularly preferably90-110° C. The pressure at the top of the column is preferably 1.0-4.7atm (gauge pressure), particularly preferably 2.0-3.7 atm (gaugepressure). The hydrogen chloride/phosgene ratio in the column is set andcontrolled by means of the excess of phosgene in the first stage of thephosgenation, the temperature at which the reaction product enters thecolumn, the column pressure and the temperature at the bottom of thecolumn. The phosgene can all be fed into the first stage of thephosgenation or only part of it can be introduced into the first stage.In the latter case, a further amount of phosgene is fed into theresidence time apparatus of the second stage of the phosgenation. Thecolumn used preferably has <10 theoretical plates. The use of a valvetray column is advantageous. It is also possible to use other internalcolumn fittings which ensure the necessary residence time for thecarbamoyl chloride dissociation and rapid and effective removal ofhydrogen chloride, for example bubble cap tray columns or distillationtrays having relatively high liquid weirs. The perforated plate columnproposed in DE-A 3 744 001 is very unsatisfactory in industry for thetask of mild carbamoyl chloride dissociation together with rapid andeffective hydrogen chloride removal and is unsuitable as residence timeapparatus for preparing a PMDI having a reduced chlorine content and areduced iodine color number because of its cocurrent principle whichinevitably leads to large liquid holdups and to greater difficulty inachieving rapid removal of hydrogen chloride.

The mixtures of diphenylmethane diisocyanates andpolyphenylpolymethylene polyisocyanates prepared by the process of thepresent invention usually have a diphenylmethane diisocyanate isomercontent of from 30 to 90% by mass, preferably from 30 to 70% by weight,an NCO content of from 29 to 33% by weight, preferably from 30 to 32% bymass, based on the weight of crude MDI, and a viscosity, determined at25° C. in accordance with DIN 51550, of not more than 2500 mPa.s,preferably from 40 to 2000 mPa.s.

Crude MDIs having such isomer and homologue compositions can be preparedby phosgenation of crude MDAs having corresponding product compositionsin the presence of at least one inert organic solvent.

Suitable crude MDAs are advantageously obtained by condensation ofaniline and formaldehyde in a molar ratio of 6-1.6:1, preferably4-1.9:1, and a molar ratio of aniline to acid catalysts of 1:0.98-0.01,preferably 1:0.8-0.1.

The formaldehyde is preferably used in the form of an aqueous solution,e.g. as a commercial 30-50% strength by mass solution.

Acid catalysts which have been found to be useful are proton donors suchas acid ion exchange resins or strong organic and preferably inorganicacids. For the purposes of the present invention, strong acids are thosehaving a pKa of less than 1.5; in the case of polybasic acids, thisvalue is that for the first hydrogen dissociation. Examples which may bementioned are hydrochloric acid, sulfuric acid, phosphoric acid,fluorosulfonic acid and oxalic acid. Hydrogen chloride in gaseous formcan also be used. Preference is given to using aqueous hydrochloric acidin concentrations of from about 25 to 33% by mass.

Suitable processes for preparing crude MDA are described, for example,in CA-A-700 026, DE-B-22 27 110 (U.S. Pat. No. 4,025,557), DE-B-22 38920 (U.S. Pat. No. 3,996,283), DE-B-24 26 116 (GB-A-1,450,632),DE-A-12,42,623 (U.S. Pat. No. 3,478,099), GB-A-1,064,559 and DE-A-32 25125.

The other starting component for preparing crude MDI is phosgene. Thegaseous phosgene can be used as such or diluted with gases which areinert under the reaction conditions, e.g. nitrogen, carbon monoxide,etc. The molar ratio of crude MDA to phosgene is advantageously selectedsuch that from 1 to 10 mol, preferably from 1.3 to 4 mol, of phosgeneare present in the reaction mixture per mole of NH₂ groups. The phosgenecan all be fed into the first stage of the phosgenation or part of itcan also be added to the residence time apparatus of the second stage ofthe phosgenation.

Suitable inert organic solvents are compounds in which the crude MDA andthe phosgene are at least partially soluble.

Solvents which have been found to be useful are chlorinated, aromatichydrocarbons, for example monochlorobenzene, dichlorobenzenes such aso-dichlorobenzene and p-dichlorobenzene, trichlorobenzenes, thecorresponding toluenes and xylenes, chloroethylbenzene,monochlorobiphenyl, alpha- or beta-naphthyl chloride and dialkylphthalates such as diethyl isophthalate. Particular preference is givento using monochlorobenzene, dichlorobenzenes or mixtures of thesechlorobenzenes as inert organic solvents. The solvents can be usedindividually or as mixtures. It is advantageous to use a solvent whichhas a boiling point lower than that of the MDI isomers so that thesolvent can easily be separated from the crude MDI by distillation. Theamount of solvent is advantageously selected such that the reactionmixture has an isocyanate content of from 2 to 40% by mass, preferablyfrom 5 to 20% by mass, based on the total weight of the reactionmixture.

The crude MDA can be employed as such or as a solution in organicsolvents. However, particular preference is given to using crude MDAsolutions having an amine content of from 2 to 45% by mass, preferablyfrom 25 to 44% by mass, based on the total weight of the amine solution.

Subsequent to the phosgenation, the excess phosgene, the hydrogenchloride and the solvent are preferably separated from the reactionproduct. To prepare a PMDI having a reduced content of chlorinatedby-products and a reduced iodine color number, it is particularlyadvantageous for the residual content of phosgene after the phosgeneremoval to be <10 ppm of phosgene. These work-up steps are carried outby generally known methods. The two-ring isomers can be separated fromthe MDI mixture by known methods such as distillation orcrystallization.

The product is then usually stabilized using an antioxidant based onsterically hindered phenols and/or at least one aryl phosphite. Thestabilizers are advantageously used in an amount of up to max. 1% bymass, preferably from 0.001 to 0.2% by mass. Examples of suitableantioxidants based on sterically hindered phenols are: styrenizedphenols, i.e. phenols which have a 1-phenylethyl group bound in the 2 or4 position or in the 2 and 4 and/or 6 positions,bis(2-hydroxy-5-methyl-3-tert-butylphenyl)methane,2,2-bis(4-hydroxyphenyl)propane, 4,4,′-dihydroxybiphenyl, 3,3′-dialkyl-or 3,3 ′, 5,5′-tetraalkyl-4,4′-dihydroxybiphenyl,bis(4-hydroxy-2-methyl-5-tert-butylphenyl)sulfide, hydroquinone,4-methoxy-, 4-tert-butoxy- or 4-benzyloxy-phenol, mixtures of4-methoxy-2- or -3-tert-butylphenol, 2,5-dihydroxy-1-tert-butylbenzene,2,5-dihydroxy-1,4-di-tert-butylbenzene,4-methoxy-2,6-di-tert-butylphenol and preferably2,6-di-tert-butyl-p-cresol.

Aryl phosphites which have been found to be useful aretri(alkylphenyl)phosphites having from 1 to 10 carbon atoms in the alkylradical, e.g. tri(methylphenyl)phosphite, tri(ethylphenyl)phosphite,tri(n-propylphenyl)phosphite, tri(isopropylphenyl)phosphite,tri(n-butylphenyl)phosphite, tri(sec-butylphenyl)phosphite,tri(tert-butylphenyl)phosphite, tri(pentylphenyl)phosphite,tri(hexylphenyl)phosphite, tri(2-ethylhexylphenyl)phosphite,tri(octylphenyl)phosphite, tri(2-ethyloctylphenyl)phosphite,tri(decylphenyl)phosphite and preferably tri(nonylphenyl)phosphite, andin particular triphenyl phosphite.

The present invention also provides a process for preparing 2,2′-, 2,4′-and/or 4,4′-MDI from the mixture comprising diphenylmethane diisocyanateand polyphenylpolymethylene polyisocyanate prepared according to thepresent invention, which comprises separating 2,2′-, 2,4′- and/or4,4′-MDI, preferably 4,4′-MDI, by distillation and/or crystallizationfrom the mixtures prepared according to the present invention.

Accordingly, the crude PMDIs prepared in this way are usually subjectedto a thermal after-treatment which can be coupled with the separation ofthe MMDI isomers. For this purpose, the PMDI is heated to 170-230° C.,preferably 180-220° C., and treated at this temperature at a pressure offrom 0.01 to 100 mbar, preferably from 0.1 to 20 mbar, for at least 5minutes and in particular from 5 to 45 minutes, if desired while passingin an amount of at most 5 standard m³/t of PMDI of an inert gas such asnitrogen, preferably at most 0.5 standard m³/t of PMDI of inert gas.

After cooling to 30-60° C., the PMDI is usually passed to intermediatestorage.

The invention is illustrated by the examples below:

EXAMPLE 1

The phosgenation is carried out using a PMDA having the followingcomposition:

viscosity at 70° C. 348 mm²/s 4,4′-diphenylmethanediamine 44.6% by mass(4,4′-MDA) content MDA content 52% by mass 3-ring-PMDA content 23% bymass N-methyl-MDA content 0.14% by mass N-formyl-MDA content 1194 ppm.

3840 kg/h of such a PMDA as a 38.7% strength by mass solution inmonochlorobenzene (MCB) are phosgenated with 26,400 kg/h of a 42%strength by mass solution of phosgene in MCB in an angle-jet chambernozzle. The reaction mixture heats up to 118° C. in the reactor of thefirst stage of the phosgenation as a result of the exothermic reactionof PMDA with phosgene and is at 92° C. on entry into a valve tray columnhaving 6 theoretical plates in the stripper section and 2 plates in theenrichment section. The column is operated at a pressure of 4.3 bar(abs.) and the composition of the bottoms is adjusted by means of theamount of steam used for heating so that the phosgene content at thebottom of the column is about 10% by mass, which corresponds to atemperature at the bottom of the column of 95-97° C. The mass ratios ofphosgene to hydrogen chloride are 14.2:1 at the bottom of the column and1.6:1 at the top of the column. The hydrogen chloride formed in thefirst stage of the phosgenation and liberated in the column from thedissociation of the carbamoyl chlorides is, together with part of thephosgene used in excess, taken off at the top at 91° C. To prevententrainment of PMDI droplets in the hydrogen chloride and phosgene gasstreams, 1350 kg/h of MCB are additionally fed in at the top of thecolumn.

The mixture leaving the phosgenation is freed of phosgene and MCB andthermally after-treated in accordance with the prior art.

The PMDI prepared in this way has the following product properties:

viscosity at 25° C. in accordance 182 mPa · s with DIN 51550 isocyanategroup content in accordance 31.5% by mass with ASTM D 1638-74 acidity inaccordance with ASTM D 1638-74 56 ppm HCl total chlorine in accordancewith DIN 35474 900 ppm HCl iodine color number¹⁾ 9.7 ¹⁾Measured using athree-filter instrument, e.g. LICO 200 (Dr. Lange)

Comparative Example 1

For comparison, the same PMDA as in Example 1 is phosgenated in the sameangle-jet chamber nozzle and the same column. 3840 kg/h of this PMDA asa 38.7% strength by mass solution in monochlorobenzene (MCB) arelikewise reacted with 26,400 kg/h of a 42% strength by mass solution ofphosgene in MCB. Likewise, 1350 kg/h of MCB are additionally fed in atthe top of the column.

The entry temperature of the PMDA/MCB stream into the angle-jet chambernozzle is selected so that the temperature of the reaction mixtureleaving the nozzle is 96° C. The reaction mixture is at 78° C. on entryinto the valve tray column. The column is operated at a top pressure of5.2 bar (abs.). At a bottom temperature set to 116° C., a temperature atthe top of 76° C. is established. The mass ratios of phosgene tohydrogen chloride are 9.2:1 at the bottom of the column and 0.95:1 atthe top of the column.

The PMDI prepared as a comparision has the following product properties:

viscosity at 25° C. in accordance 197 mPa · s with DIN 51550 isocyanategroup content in accordance 31.8% by mass with ASTM D 1638-74 acidity inaccordance with ASTM D 1638-74 197 ppm HCl total chlorine in accordancewith DIN 35474 1900 ppm HCl iodine color number¹⁾ 15

We claim:
 1. A process for preparing mixtures comprising diphenylmethanediisocyanates and polyphenylpolymethylene polyisocyanates having areduced content of chlorinated by-products and a reduced iodine colornumber, said process comprising reacting diphenylmethanediamines andpolyphenylpolymethylenepolyamines (PMDA) with phosgene in the presenceof at least one inert organic solvent in a two-stage reaction comprisingfirst and second phosgenation stages, wherein the mass ratios of saidphosgene to hydrogen chloride in a residence time apparatus of thesecond phosgenation stage are at the same time 10-30:1 in the liquidphase and 1-10:1 in the gas phase.
 2. A process as claimed in claim 1,wherein the first phosgenation state comprises using a static mixerhaving a mix exit temperature of 80-120° C.
 3. A process as claimed inclaim 1, wherein the second phosgenation stage comprises using a columnhaving <10 theoretical plates.
 4. A process as claimed in claim 3,wherein the column is operated in countercurrent fashion.
 5. A processas claimed in claim 3, wherein the column is a valve tray column.
 6. Aprocess as claimed in claim 3, wherein the column is a bubble cap traycolumn.
 7. A process as claimed in claim 3, wherein the column hasdistillation trays having relatively high liquid weirs.
 8. A process asclaimed in claim 2, wherein the PMDA concentration in the inert solventin the stream to the static mixer is at most 44% by mass.
 9. A processas claimed in claim 3, wherein the temperature at which the mixture fromthe first phosgenation stage enters the column is 80-120° C.
 10. Aprocess as claimed in claim 3, wherein the temperature at the bottom ofthe column is 80-120° C.
 11. A process as claimed in claim 3, whereinthe pressure at the top of the column is 1.0-4.7 atm (gauge pressure).12. A process as claimed in claim 9, wherein the temperature at whichthe mixture from the first phosgenation stage enters the column is82-117° C.
 13. A process as claimed in claim 10, wherein the temperatureat the bottom of the column is 90-110° C.
 14. A process as claimed inclaim 11, wherein the pressure at the top of the column is 2.0-3.7 atm(gauge pressure).